What question about Being did Heidegger raise in Being and Time?

In his seminal work Being and Time (Sein und Zeit), Martin Heidegger raised the Seinsfrage, or the "question of Being," which serves as the fundamental inquiry of his ontology.12

Heidegger formulated this question as: "If Being is predicated in manifold meanings, then what is its leading fundamental meaning? What does Being mean?" In other words, he sought to identify the most fundamental kind of Being that could be predicated of all things, rather than focusing on the properties of individual beings.23

To address this question properly, Heidegger argued that it was necessary to first conduct a preliminary phenomenological investigation into the Being of the human individual, whom he termed Dasein ("being-there"). By analyzing the fundamental structures of Dasein—such as its condition of "Being-in-the-world"—Heidegger aimed to move beyond traditional Cartesian conceptions of the human as a mere thinking subject, which he believed had obscured a proper understanding of Being.123